This invention relates to a hydrostatic drive arrangement for a hydrostatically driven vehicle having left and right hydrostatic transmissions.
In the hydrostatically driven vehicle, a difference in efficiency between the left and right hydrostatic transmissions will result in a difference in the number of output revolutions between the left and right hydraulic motors which in turn causes a turning of the vehicle to either side when it is making a straight drive on. Consequently, a problem will occur that in order to prevent the turn of the vehicle to either side it becomes necessary, in operation, to make a steering operation which is otherwise unnecessary.
To eliminate the above-mentioned problem, such proposals as disclosed in the U.S. Pat. Nos. 3,908,374 and 3,655,004 have been made; however, both of them have the following disadvantages and cannot be recognized as complete or satisfactory ones.
First, in vehicles provided with a hydrostatic drive arrangement as disclosed in the U.S. Pat. No. 3,908,374, there is a possibility that the following phenomenon occurs.
If a steering operation is made, for example, to the left transmission when synchronizing valve is under bypass condition (that is; the circuit pressure is lower than a preset value), the hydraulic fluid will flow from the higher fluid pressure conduit of the right transmission into the higher fluid pressure conduit of the left transmission. Such a condition prevents the steering operation of the vehicle, and so it becomes impossible to make a smooth steering of the vehicle.
If the steering operation is continued further, the pressure within the conduit of the right transmission on the higher pressure side thereof will rise due to the resistance caused by the left transmission thereby shifting the synchronizing valve to a blocking position. As a result, the hydraulic fluid flow from the right transmission to the left transmission is interrupted so that the vehicle will commence a quick turn.
Accordingly, the above-mentioned prior art hydrostatic drive arrangement has been disadvantageous in that at the time of commencement of steering operation the vehicle cannot be turned as desired, whilst when the steering operation is continued further beyond a certain extent the vehicle will turn suddenly, and such unexpected turn of the vehicle is not preferrable for the operator.
Whilst, in the hydrostatic drive arrangement disclosed in the U S. Pat. No. 3,655,004, such a phenomenon will not occur and straight drive and steering of the vehicle can be conducted smoothly; however, it has been disadvantageous in that there is provided a clutch for mechanically connecting and disconnecting the output shafts of the hydraulic motors thus making the construction thereof complicated and increasing the cost thereof.